linux-kernel - Re: [PATCH v4 5/7] mm: Remember young/dirty bit for page migrations

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Date:   Sun, 11 Sep 2022 16:48:28 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Alistair Popple <apopple@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v4 5/7] mm: Remember young/dirty bit for page migrations

On Thu, 11 Aug 2022 12:13:29 -0400 Peter Xu <peterx@...hat.com> wrote:

> When page migration happens, we always ignore the young/dirty bit settings
> in the old pgtable, and marking the page as old in the new page table using
> either pte_mkold() or pmd_mkold(), and keeping the pte clean.
> 
> That's fine from functional-wise, but that's not friendly to page reclaim
> because the moving page can be actively accessed within the procedure.  Not
> to mention hardware setting the young bit can bring quite some overhead on
> some systems, e.g. x86_64 needs a few hundreds nanoseconds to set the bit.
> The same slowdown problem to dirty bits when the memory is first written
> after page migration happened.
> 
> Actually we can easily remember the A/D bit configuration and recover the
> information after the page is migrated.  To achieve it, define a new set of
> bits in the migration swap offset field to cache the A/D bits for old pte.
> Then when removing/recovering the migration entry, we can recover the A/D
> bits even if the page changed.
> 
> One thing to mention is that here we used max_swapfile_size() to detect how
> many swp offset bits we have, and we'll only enable this feature if we know
> the swp offset is big enough to store both the PFN value and the A/D bits.
> Otherwise the A/D bits are dropped like before.
> 

There was some discussion over v3 of this patch, but none over v4.

Can people please review this patch series so we can get moving with it?

Thanks.